Dancing with Light: Discrete Pi-Conjugated Chromophores for Solar Fuel Cell and Electrochromic Applications
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Molecular engineering plays a pivotal role in answering fundamental scientific questions aimed at understanding the relationship between a material and its potential technological application. This dissertation focuses on outlining structural design strategies implemented to create new classes of discrete π-conjugated organic chromophores that target either dye sensitized photoelectrosynthesis cells or electrochromic devices. Three major questions are explored in this research: 1) How can chemical structures be manipulated to effectively fine-tune the chromophores' redox potentials? 2) How do steric interactions and electron densities affect the optical properties of oxidized species? 3) What design paradigms, if any, can be elucidated from the trends observed in the experimental data and theory? As a preliminary evaluation of their potential utility, each investigation employs spectroscopic and electrochemical measurements and computational calculations to characterize the chromophores' redox and optoelectronic properties. First, this work introduces a new class of donor-acceptor-donor (DAD) molecules with high oxidation potentials that target hole injection into PbTiO3, an alternative p-type semiconductor to the state-of-the-art NiO. Differential pulse voltammogram scans showed that the chromophores' redox potentials were systematically tuned as a function of acceptor strength. An absorbed-photon to current conversion efficiency of 54% was reported, which is indicative of efficient hole injection. This work demonstrates how acceptor and donor choices can dictate energy alignments which strongly influence photon-conversion-efficiencies. Furthermore, two benzothiadiazole-type DAD constitutional isomers were synthesized, and their charge transfer processes at the photoanode and photocathode were investigated. Structures analyzed by single-crystal x-ray diffraction (SC-XRD) showed significant torsional differences of 44ᵒ and 177ᵒ between the acceptor and donor aryl rings of the two molecules. Photoelectrocatlytic measurements of the chromophore-catalyst assemblies demonstrated that both sensitizers generated similar Faradaic efficiencies. However, only the planar analog was suggested for further studies because it has the advantage of binding to ZrIV ions, a capability needed to build complex photoelectrode layers. These results illustrate how structural conformation can impact both charge dynamics and molecular assemblies. This dissertation also examines a family of seven anodically coloring (ACE) thiol-ether-ethylene(dioxythiophene)-(BEDOT)-alkoxyphenylene (BEDOT-X) molecular electrochromes. Time-Dependent Functional Theory was instrumental in simulating the optical properties (high oscillator strength), and geometries (strong Van der Waals forces and planar conformation) used to guide the synthetic targets. BEDOT-dimethoxyphenylene (BEDOT-2,5-DMOB) was also predicted to have the highest color saturation and was chosen as the primary candidate for synthesis, followed by two others. Spectroelectrochemistry conducted in an optically thin layer electrode, and chemical dopant measurements revealed similar two-band absorption profiles across the set, with λmaxpeak1 ~600 nm and λmaxpeak2 ~1200 nm. In comparison to previous ACE families, the lower-energy band is significantly redshifted into the NIR. The one band absorption in the visible range indicates that each oxidized species only exhibits a single hue and consequentially appears blue in color. Colorimetry analysis of the doped solutions confirms this observation with negative b* ~ -40 to -64 and low a* ~14 to 38 values. Furthermore, oxidized BEDOT-2,5-DMOB displayed the most narrow and intense absorbance peak due to steric-induced conformational locking, which unexpectedly also influenced its crystal packing structure. SC-XRD revealed that the crystal shows an unusual plastic-like deformation behavior capable of reversibly bending in response to an applied force by a metal probe. This phenomenon was not observed in the methoxy analog, which suggests that the additional O-S interactions in BEDOT-2,5-DMOB impact the order-disorder transition. In summary, this research shows that color saturation and hue in cation radical absorptions can be controlled by steric interactions, electron densities, and conjugation length. In addition to its ACE application, the discovery of plastic crystals opens new possibilities to fully characterize the mechanical and thermal properties in the BEDOT family, which may have implications for mechano-adaptable technologies. In a final study, 7-(hexylthio)-2-(3,4-ethylendioxy thiophene))benzyl-phosphonic acid was synthesized and tethered onto a nano-indium tin oxide electrode. Electrochemical and optical measurements demonstrate, in a proof-of-concept, the potential deployment of discrete ACE molecules into solid-state devices.